Television system



Nov., 15, 1938. A. v. BEDFORD TELEVISION SYSTEM Filed June 9, 1954 2 Sheets-Sheet 1 INVENTOR im Vfmi;

NOV. 175, 1938. A V, BEDFORD '2,137,010

TELEVISION SYSTEM FiledJune 9, 1934 2 Sheets-Sheet 2 D VOL TH GE voL'rs VL TS GRID PLATE CURRENT INVENTOR Alda VBedford Patented Nov. 1 5, 1938 TELEVISION SYSTEM Alda v. Bodfora, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 9, 1934, Serial No. 729,730

3 Claims. (Cl. 178--7.2)

My invention relates to television systems and particularly to television systems which comprise a cathode-ray transmitter tube and an electric discharge tube generator of defiecting and synchronizing impulses.

It has been found, in television systems utilizing vacuum tube impulse generators, such as generators of the type disclosed in my copending application, Serial No. 726,258, filed May 18, 1934, where pictures are transmitted from a motion picture film, that it is difficult to keep the pic ture properly framed. 'I'his difficulty is caused partly because of slight variations in the power line frequency, but mainly because of a tendency of the impulse generator frequency to drift,

I have discovered that the difiiculty in keeping the picture framed may be overcome by operating the impulse generator in step or locked in with the alternating current power source which supplies power for the film drive.

Operating the impulse generator in step with the power line has a further advantage in that the frequency of the impulse generator output is maintained substantially constant, since the frequency variations of the power line are so slight that it is possible to use a television receiver which does not require a speed-control adjustment. In other Words, if all substantial drift in the impulse frequency is prevented, there is no need for an adjustment on the receiver for changing the frequency of the vertical defiecting oscillator to keep it under the control of the vertical synchronizing impulses.

An object of my invention is to provide an improved television system in which pictures transmitted from a lm are properly framed.

More specifically, an object of my invention is to provide a. television system, utilizing an electric discharge tube impulse generator, in which pictures transmitted from a film are always properly framed.

A further object of my invention is to provide a television transmitter in which the frequency of an electric discharge tube impulse generator is held substantially constant.

A further and more specific object of my invention is to control the frequency of an electric discharge tube impulse generator with respect to the frequency of the current from a power line or the like.

In one embodiment of my invention, a vacuum tube impulse generator is utilized which has an output suitable for interlaced scanning as described in my above-mentioned copending application. A motion picture film having a picture thereon to be transmtted is moved into position in a lm gate by means of a standard intermittent mechanism which is driven by a synchronous motor connected to a 60 cycle power line.

Electrical impulses from the output of the impulse generator and electrical impulses derived from the power line are fed into a common control circuit which changes the frequency of the main oscillator of the impulse generator in response to any phase shift of the impulses in said common circuit.

Other objects, features, and advantages of my invention will appear from the following description taken in connection with the accompanying drawings, in which Figure 1 is a View of a television transmitter embodying my invention,

Fig. 2 is an end view of a shutter disc shown in Fig. 1,

Fig. 3 is a group of curves which are referred to in explaining my invention,

Fig. 4 is a circuit diagram of an amplifier employed in the system shown in Fig. 1,

Fig. 5 is a set of curves which are referred to in explaining the operation of my control circuit, and

Fig. 6 is a chart illustrating the operation of the picture projector and scanner.

Referring to Fig. 1, a. television transmitter system is illustrated which includes a cathode ray transmitter tube I of the type described in an article by Dr. V. K. Zworykin which appears in the January, 1934, issue of The Journal of The Franklin Institute. 'Ihis tube comprises an evacuated envelope 3 having an electron gun 5, a focusing and accelerating anode 1 and a mosaic 9 of light sensitive elements thereon. Defiecting means, such as defiecting coils II and I3, are provided for defiecting the electron beam horizontally and vertically, respectively, in order to scan the mosaic 9 with the beam.

To produce the desired current flow through the defiecting coils II and I3 for defiecting the electron beam, an electric discharge tube impulse generator is provided. This generator consists of a main oscillator I5 from which are derived electrical impulses of the desired frequency for horizontal deflection and also electrical impulses of the desired frequency for vertical deflection. The main oscillator I5 may be a blocking oscillator consisting of an electrical discharge tube I1 having a cathode I9, a control grid 2l and an anode 23. The control grid 2| is connected through a variable grid leak resistor 25 and through a resistor 21 in the control circuit (which will be described hereinafter) to ground and through ground to the cathode I9.`

The anode 23 is maintained at a positive potential by means of a suitable voltage source such as a battery 29 which has its negative terminal connected to the cathode l5 and its positive terminal connected through a resistor 3| and the primary winding 33 of a transformer 35 to the anode 23. A condenser 31 is connected in series with the secondary 33 of the transformer 35, the condenser 31 and secondary 39 being connected between the control grid 2l and the cathode I9.

The transformer windings 33 and 35 are so phased that sfn increase in plate current in the tube I1 causes the control grid 2l to become more positive whereby grid current flows into the grid condenser 31 to charge it in such a direction as to apply a negative bias to the control grid 2l. The plate current finally approaches its saturation value so that its rate of increase becomes less, the voltage fed back to the grid decreases whereby the plate current decreases, thus reversing the phase of the voltage applied to the control gri4 through the transformer 35 and biasing the tube I1 beyond cut-0H. During the time the grid has been positive, the condenser has been charged sufficiently to hold the tube biased beyond cut-off and no further oscillation takes place until the charge has leaked off the condenser 31 through the grid leak resistor 25 sumciently to lower the gridvoltage to a value which again permits flow of plate current.

Ihe voltage impulses produced by the oscillator l5 are impressed upon a chain of frequency dividers Il, 43, 45, and 41, each of which may be a blocking oscillator operating the same as the main oscillator l5 but at a lower frequency. Each oscillator is locked in step with the oscillator of next highest frequency. The circuit of the frequency divider 4I is illustrated in order to illustrate the invention more completely but a description of the circuit is not believed to be necessary in view of the description of the main oscillator circuit I5.

The frequency of the main oscillator output is divided in two steps of three and two steps of five, as indicated on the drawings to produce voltage impulses occurring at the rate of 48 per second, this being the desired vertical deflecting or frame frequency. The 48 cycle impulses are supplied to the input circuit of a saw-tooth Wave generator 4I which converts them into voltage impulses of the necessary shape for causing a flow of current, having a saw-tooth wave shape, through the deflecting coils I3.

'I'he horizontal defiecting impulses are obtained by impressing a portion of the output energy of the main oscillator I5 upon the input circuit of a frequency divider 5i which divides the frequency of the main oscillator output by two to produce 5400 impulses per second. The output of the frequency divider 5| is impressed upon a saw-tooth wave generator 53 which has its output connected across the horizontal deecting coils Il for causing a iiow of saw-tooth wave current therethrough.

It will be noted that the vertical deilecting and horizontal deflecting frequencies are such as to cause 1121/2 lines per picture frame, in order to provide interlaced scanning as described and claimed in my above identified copending application. Although my invention is described in connection with an interlaced scanning system, it should be understood that it is of general application.

My improved television system is especially designed, as will .be explained hereinafter, for the transmission of pictures on a motion picture film, such a film being indicated generally at 55. The picture frames are moved intermittently into position in a film gate 51 by means of a standard intermittent 59. 'I'he intermittent 59 is driven through a gear box 6I by a synchronous motor 53 at the standard speed whereby 24 pictures per second are projected upon the mosaic 9. A shutter disc 55 is also driven by the synchronous motor 53 through the gear box 5i, it being rotated at 24 revolutions per second.

As shown in Fig. 2the shutter disc 55 has two openings 61 and 69 therein which are diametrically opposite each other for the purpose of cutting of! the light projected toward the mosaic periodically, as will be more fully explained hereinafter.

For reasons previously mentioned, and particu. larly in order to keep the picture properly framed, a control circuit 1i is provided for maintaining the impulse generator locked in with the 60 cycle power line 13 to which the motor 53 is connected. The control circuit includes an electric discharge tube 15 having a cathode 11, a control grid 19 and an anode 8l. The tube 15 is so adjusted that it functions as a distorting amplifier or detector. In one specific embodiment this is accomplished by maintaining the control grid 19 at a high negative bias by means of a biasing source, such as a battery 83, the negative bias being high enough to bias the tube beyond the cut-off potential. 'I'he anode 8| is maintained at a positive potential by means of a source of potential, such as a battery 85, having its negative terminal connected to the cathode 11 and its positive terminal connected to the anode 8l through the resistor 21. The resistor 21 is shunted by a filter condenser 81.

It will be noted that the resistor 21 is connected in series with the grid leak resistor 25 of the blocking oscillator l5 whereby any change in voltage drop across the resistor 21 causes a. change in the bias on the control grid 2l of the blocking oscillator. 'I'he filter condenser 81 has sufficient capacity to integrate current impulses and cause a substantially steady flow of current through the resistor 21.

The current flow through the control circuit resistor 21 is caused to vary in response to a phase shift between frequency of the 60 cycle current from the power line 13 and the frequency of the impulse generator output by impressing the impulses from both the power line and the impulse generator upon the input circuit of the control tube 15.

In order to effect the desired control, it is desired, in the specific embodiment illustrated, that the two groups of control impulses occur at the same frequency. Therefore, since current from the power line has a frequency of 60 cycles and the impulses from the vertical deecting circuit occur at the rate of 48 per second, the 48 cycle impulses are impressed upon a frequency divider 89 which lowers the frequency to 12 impulses per second. The output of the frequency divider 89 is impressed upon a wave shaping amplifier 9| and through a coupling condenser 93 upon the control grid 19 of the tube 15.

Current from the 60 cycle line 13 is impressed upon a frequency divider 95 which divides the frequency by five to produce impulses occurring at the rate of 12 per second. Both the frequency divider 95 and the frequency divider 89 `particularly when the may be blocking oscillators similar to the ones described above. The impulses appearing in the output circuit of the frequency divider 95 are impressed upon a wave shaping amplifier 91 from lthe output of which they are fed into the input 4circuit of the control tube 15 through a transformer 99 in such phase that they add with the 12 cycle impulses from amplier 9|.

It should be understood that the wave shaping amplifiers 9| and 91 may be omitted, if desired, distorting or detector tube 15 is biased to cut-olf, as described. Also, if

- the power line or other control source provides current at the proper frequency, the sine wave current therefrom may be impressed directly upon the control tube circuit 1| through the transformer 99.

The operation of the control circuit 1| will be better understood by referring to Fig. 3 which shows the wave shape of the voltage impulses appearing in the output circuit of a frequency divider where it is a blocking oscillator. The voltage waves indicated at and |03 are typical of the voltage impulses generated by a blocking oscillator. It will be noted that each wave includes a sharp positive impulse, this impulse being the one which is used for control purposes. The wave shaping amplifiers 9| and 91 cut od the portion of the voltage wave which appears below the dotted line |05. A preferred circuit for these wave shaping amplifiers is shown in Fig. 4.

Referring to Fig. 4, the amplifier consists of two resistance coupled amplifier stages, the first stage comprising an electric discharge tube |01 which is biased beyond the cut-off potential by means of a biasing battery |09. In the particular embodiment illustrated, the second amplier stage includes an electric discharge tube which is biased to function as a non-distorting amplifier. 'Ihe first amplifier stage |01 permits the passage of only the positive peak of the blocking oscillator voltage wave, that is, the portion of the wave above the dotted line |05, while the second stage simply amplifies this impulse and reverses its phase so that it appears in the output of the amplifier as a positive voltage impulse.

The two voltage impulses which are supplied from the wave shaping amplifiers 9| and 91 appear in the input circuit of the control tube 15 as shown at ||3 and ||5 in view a of Fig. 5. These voltage waves add to give a resultant wave ||1 which is shown in view b of Fig. 5.

The grid lvoltage-plate current characteristic of the tube 15 is shown at ||9 in view c of Fig. 5. Following conventional graphical construction, it is seen that the voltage wave ||1 impressed upon the input circuit of the tube 15 causes a flow of plate current in the output circuit of the tube which has the wave shape indicated at |2|. These impulses |2|, which occur in the output circuit of the tube 15 at the rate of 12 per second, charge the filter condenser 81 to produce the substantially steady flow of current through the resistor 21 previously referred to.

Assume that either the frequency of the main blocking oscillator i5 or the frequency of the power line 13 changes to cause the voltage impulses to move farther apart as indicated by the dotted line curve |23 in Fig. 5. The two control impulses now add to give the resultant voltage shown by the dotted line curve |25 in view b of Fig. 5, and this voltage produces a current flow in the output circuit of the control tube 15 which is represented by the dotted line curve |21 in view d of Fig. 5. It is evident that, as a result of the frequency shift, less energy is supplied to the filter condenser 91 and there will be a corresponding reduction in current flow I through the resistor 21 whereby the bias voltage on the control grid 2| of the blocking oillator tube |1 will be changed and the frequency of the blocking oscillator l5 will be corrected.

In order to explain more in detail how the frequency of the blocking oscillator I5 is corrected, it will be assumed that the voltage or' control impulses ||3 and H5, shown in Fig. 5, are the impulses derived from the power line and from the oscillator, respectively. Also, assume that the oscillator frequency decreases to shift the oscillator control impulse to the position shown by the dotted curve |29. As previously explained, this causes less current to flow through the resistor 21 whereby the negative potential 20 applied to the control grid 2| of the oscillator I5 is decreased to increase the oscillator frequency to its original value. Attention is called to the fact that the oscillator control impulse is not brought back to its original phase relation with the power line control impulse H3; instead, an equilibrium point is reached where a new phase relation between the control impulses causes the proper bias to be applied to the oscillator.

It will be understood that the frequency of the main oscillator I5 may be controlled by the control circuit 1| in various other ways than the particular one illustrated. For example, the voltage drop across the resistor 21 may be utilized to change the potential on the anode 23 of the blocking oscillator tube |1 or it may be uti--v lized to control the voltage on a second grid electrode, not shown. Also, the control impulses applied to the control circuit need not have the same frequency so long as they bear a. whole number multiple frequency relation to each other, as in the case of one group of impulses occurring at twice the frequency of the other group.

It will be apparent from the foregoing description that the control circuit` 1| and its associated circuits causes the mosaic 9 to be scanned synchronously with the projection of the picture frames thereon. The picture projection! and scanning operation is illustrated on the chart in Fig. 6. Starting with the condition just as the shutter opening 51 rotates opposite the picture projector, the picture is projected upon the mosaic 9 at the time A and remains thereon until the time B, at which time the shutter 55 intercepts the beam of light from the projector. During this time, the cathode ray has been brought to its starting position for scanning and the film has been stationary. At the time B, the cathode ray starts scanning the mosaic and completes the scanning at the time C, the film being stationary during this scanning period and there being no light upon the mosaic.

At the time C, the second shutter opening 99 rotates into position opposite the projector to permit the picture to be again projected upon the mosaic until the time D. at which time the light is again intercepted by the shutter 55. At this time, the cathode ray again begins to scan the mosaic 9, the mosaic again being dark. Up

to this time, the ilm has remained stationary.

Beginning at the time D, the intermittent 59 starts to move the next picture frame into position, At the time E, the next picture frame has been moved into position and the intermittent stops. I'he scanning of the dark mosaic con- 75 tinues until the time A. ai' which time the first shutter opening 51 again moves into position opposlte the projector and the second picture frame is projected upon the mosaic.

'I'he picture signals appear across a resistor |29 connected between the mosaic 9 and the second anode 1 and are impressedv upon the picture signal amplifier III which amplifies them and impresses them upon a radio transmitter |33 or upon a transmission line (not shown).

In order to maintain the scanning at the receiver synchronized with the scanning at the transmitter, the horizontal synchronizing impulses are impressed through a wave shaping amplifier |35 upon a suitable point in the picture signal amplifier |3I. Likewise the vertical defecting impulses are impressed through a wave shaping amplifier |31 upon a suitable point in the picture signal amplifier |3I. Preferably, the horizontal and vertical synchrozing impulses are impressed upon such a stage in the amplifier |3| that they appear at the receiver with a polarity opposite to the polarity of the picture signals in the white direction whereby the synchronizing signals and picture signals may be more easily separated.

The wave shaping amplifiers |35 and |31 may be the same as the amplifier shown in Fig. 4 except that it is preferred to adjust the plate resistor of the first tube such that. the signal strength on the grid of the second tube I I I causes the grid to cut off the plate current of the second tube near the peaks of the negative swing. With the Wave shaping amplifiers so adjusted, the lower portion of each blocking oscillator impulse is cut off as indicated by the dotted line |05 in Fig. 3 and the peak of the impulses is also cut off by the second ampliiier stage as indicated by the dotted line |39. By so shaping the synchronizing impulses before they are impressed upon the picture signal amplifier |3|, interference with the picture signals is avoided. It will be understood that in accordance with usual practice, the horizontal synchronizing impulses are transmitted at the end of each scanning line while the vertical synchronizing impulses are transmitted at the end of each picture frame whereby the picture signals and the synchronizing signals are not transmitted simultaneously.

From the foregoing description it will be understood that various modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be imposed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim as my invention:

1. In a television system including a cathoderay transmitter tube having a mosaic of electron emissive capacity elements therein, means for generating horizontal and vertical deiiecting impulses for said tube, said means including an oscillator, means for obtaining a voltage wave from an alternating current source, means for moving a film having a picture thereon to be transmitted, said means including a synchronous motor connected to said source, means for projecting the pictures of said film upon said mosaic, means for obtaining control impulses having a fixed frequency relation to said vertical deflecting impulses, means for impressing said control impulses and said voltage wave upon a common circuit, means for producing a substantially steady current having a value depending upon the phase relation of said control impulses and said wave in said common circuit, and means for controlling the frequency of said oscillator in accordance with the value of said substantially steady current.

2. In a television system including a cathoderay transmitter tube having a mosaic of electron emissive capacity elements therein, means including an oscillator for generating comparatively high frequency horizontal defiecting impulses and comparatively low frequency vertical defiecting impulses for defiecting the cathode ray in said tube, means for obtaining comparatively low frequency control impulses from said oscillator, means for obtaining periodic voltage pulses from an alternating current power source, means for intermittently moving a film having a picture thereon to be transmitted, said means including a synchronous motor connected to said power source, means for projecting a frame of said `film upon said mosaic when said film is stationary, and means for controlling the frequency of said oscillator in accordance with the phase difference of said control impulses and said voltage impulses.

3. In a television system in which a picture is to be projected upon an electron emissive screen surface and in which an electron stream is to be deflected both vertically and horizontally, means for generating horizontal and vertical defiecting impulses for producing said deflection, said means including an oscillator, means for obtaining a voltage wave from an alternating current source, means connected to the power supply for moving a film having pictures thereon with a motion having a predetermined relation to the frequency of said alternating current, means for projecting the pictures of said film upon said screen surface, means for obtaining control impulses having a fixed frequency relation to said vertical deecting impulses, means for impressing said control impulses and said voltage wave upon a common circuit, means for producing a substantially steady current having a value depending upon the phase relation of said control impulses and said wave in said common circuit, and means for controlling the frequency of said oscillator in accordance with the value of said substantially steady current.

ALDA V. BEDFORD. 

